1. Field of the Invention
This invention relates to a circuit arrangement for suppressing magnetic induction noise in a pair of parallel conductors due to an alternating magnetic field, when a signal source and a signal receiver are connected by parallel conductors in an alternating magnetic field.
2. Description of the Prior Art
A signal source is connected normally through a pair of conductors to a signal receiver. When a part of the pair of conductors is located in an alternating magnetic field, an alternating voltage, which is proportional to the product of the intensity of the alternating magnetic field and the area projected by the conductors in a plane perpendicular to lines of magnetic force, appears at the input terminal of the signal receiver. Sine this alternating voltage constitutes part of the noise, if the amplitude of the alternating magnetic field is great and the output voltage of the signal source is small, it adversely affects the signal-to-noise ratio, when signals are detected at the signal receiver end of the conductors.
In order to suppress this sort of noise, the conductors normally form a twisted pair, so that alternating voltages induced is successive small loops formed in a projection plane, which is perpendicular to lines of magnetic force, by the intertwisted conductors, compensate each other. In this method, noise due to an alternating magnetic field can be reduced considerably. However, this method cannot be applied by printed circuits. Another method for reducing this sort of noise consists in shielding the conductors with a sheath made of a high permeability substance. In this method, the sheath should be thick enough to appreciably reduce the noise. This method is also disadvantageous in that it is hardly applicable to printed circuits.
In a magnetic bubble memory device, a magnetic bubble element is located in a rotating magnetic field produced by two coils, the magnetic fields of which are perpendicular to each other and parallel to the magnetic bubble element. The electric connection of the magnetic bubble element to an external circuit is secured by several pairs of conductors of a printed circuit disposed on a flexible film. If the two conductors of each pair were completely parallel to the magnetic fields everywhere, there would be no magnetic flux intersecting any closed circuit. Hence, no magnetic induction noise would be induced. However, in practice, in some locations, for example, where the printed circuit for electric connection to the external circuit quits the rotating magnetic field generating coils or where the printed circuit is connected to the magnetic bubble element, there exists more or less magnetic flux intersecting the two conductors. This flux induces magnetic induction noise. Since signal voltages coming from the magnetic bubble element are about 1mV or less, while the sum of the magnetic induction noise at 100 kHz is, in general, several millivolts, the signal and noise levels are in the same order of magnitude.
The noise level induced at the connection of a pair of conductors in a printed circuit to a magnetic bubble element, where the latter is disposed on a flexible film, and the pair of conductors in the printed circuit are so connected to the magnetic bubble element by two bonding leads that the bonding leads are as short as possible, can be calculated as follows. The rotating magnetic field is parallel to the pair of conductors and perpendicular to the side surfaces of the magnetic bubble element. Therefore, magnetic induction noise is induced in the closed circuit including the two bonding leads. Its value can be determined by using the following formula: EQU V = .mu..sub.o S H .omega.,
where .mu..sub.o is the permeability of air, S is the area of a closed loop in a projection plane intersecting the rotating magnetic field, and H and .omega. are the intensity and an angular frequency of the rotating magnetic field, respectively. For a distance of 0.5 mm between the bonding leads, a magnetic bubble element height of 0.5 mm, a frequency and an amplitude of the rotating magnetic field of 100 kHz and 40 Oe, the noise level is 0.63 mV. Under the same conditions, measured values were between 0.5 and 1mV. Thus, the noise level induced at the connection of a pair of conductors to a magnetic bubble element disposed on a flexible film is not at all negligible with respect to signals.